Struggling with loft command

[Edit3D.studio]

Yesterday I quit work punching my desk with incredible power due to loft function, today I'm 100% relaxed and I have recorded a sample of the issue..

[davebYYPCU]

And the problem is?

You have an expected result.

[Edit3D.studio]

In the sample profile 1 / 2 / 3 are selected in order but you cant select the guiderail so you cant loft. If you select the guiderail between the profiles selection it works, if it's normal ok I tought it's a bug. Yesterday it took me 4 hours to discover this issue, maybe it's just about me.

[davebYYPCU]

Ok, Loft is finicky, and your end result, is an expected result, 

The order of selecting profiles was demonstrated, need to sequential, 

Not being able to select the rail, maybe hold the left mouse button longer for the "Select other" drop down, 

if that doesn't work then I think you are right and found a bug.

Might help....

[beresfordromeo]

Hi there

There is something a bit strange and in some ways counterintuitive about lofting. I could be really wrong about this but what appears to happen (your video demonstrates this I think) is that when you select the profiles before the rail the loft cannot use the rail because it has effectively already performed the loft and the resultant geometry means that the rail is no longer the same shape. In this case the rail line you are trying to select is no longer continuous and does not intersect the profiles. That is to say that the loft is being performed as you select the profiles or rails and resultant calculations are being performed on the new geometry not the original. I hope this makes sense.

I think that this could be cured by the developers by allowing you to select the rail before the profiles though I don't know if this could have some other undesired effects. If you look at the screencast I have attached however you can see a very crude workaround for this problem and maybe one that suggests that the tool could be altered to prevent this behaviour. 

What I have done is to select the profiles (the order is important because you are still bound by the timeline aspects  of the loft) then a path for the rail (rail 1) which fails but is a path nonetheless, then the desired path (rail 2) and after that deleting rail 1.

Another workaround is to select 1 profile then the rail and then delete the original profile which effectively allows the rail to be selected first (though in a very round about way) which is why I think the tool could possibly be altered to allow rail preselection before profile selection. Perhaps that is an idea for the team.

I guess that in the end the most efficient way in your case is to choose 1 profile then the rail then the rest of the profiles, but then you have already worked that out so all is well there.

Anyway hope this helps.

[jeff_strater]

The order of selecting rails vs profiles should not matter.  You can select the rails first, then the profiles, or the other way around.  Or, you can mix and match.  One profile, one rail, another profile, etc.  If there are cases where that does not work, then that's a bug.

What does matter is the order the profiles are selected.  There is a reason for this.  In a case like this, where there are 3 profiles, the order of selection is used to disambiguate between several possible results.  If you pick that "middle" section last, loft will think you are trying to create a different loft - in your case, one that goes from the left horizontal profile, then to the right, then to the middle.  That loft might not be possible, but that's what you are telling it to do.

Finally, is loft even the right tool to use here?  Are all the profiles the same size?  If so, I'd just use Sweep on this case and not bother with Loft at all.

Jeff

[beresfordromeo]

Hi Jeff 

Many thanks for your help here.

I have recorded a screencast to demonstrate why selecting a rail without a profile fails. Hopefully this is just because I am missing something. 

This also happens if you project the rail path into a sketch.

I quite agree about the sweep but I think in the OPs case the middle profile (profile 2) is not the same size as the others. If the profiles are the same size in my example this is only by coincidence as I did not bother to dimension them.

[laughingcreek]

It's sometimes hard to tell design intent, but from what it looks (to me) like your trying to achieve, the easiest option is a simple extrude cut.

Like this-

[jeff_strater]

Ah, good point.  That workflow does seem to be problematic.  I've gotten it to work sometimes, but not always.  So, I'll amend my statement:  You do need to select a profile first, but thereafter, you should be able to mix and match the order of profiles and rails.

Thanks for sharing the video!

Jeff

[Edit3D.studio]

My post was just a sample of I similar issue I had in another part where I'm working on. I tought it was about that solid but it wasn't

[laughingcreek]

My bad.  Sometimes interpret posts as "how do I?" instead of "Why?"

So I got the loft to perform the same bad behavior you demonstrated.  Interestingly, if you select "new body" instead of "cut" you can select that rail in the way you would expect to be able to.  I guess since it's cutting through the rail in the preview, it's interpreting that edge as an invalid input.  I would also consider this a bug.  certainly annoying. @jeff_strater, what do you think?

[jeff_strater]

Hi @laughingcreek.  Good catch.  I think your analysis is correct.  I would call this a bug.  I expected that if I held down CTRL/CMD, it would roll back the preview, and let me select the rail, but that didn't work.  I'll file that bug.

There is a workaround.  Create another sketch, the use Include 3D Geometry from the edge, and use that as the rail

Thanks for the debugging!

Jeff

[edit]  This is a known bug, and is already being worked on!  

[laughingcreek]

Another work around that works is to select the rails while the loft is set to "new body", and then switch it back to "cut" right before ending the command

[beresfordromeo]

That is a great workaround.

And perhaps it supports the idea that it could be easily corrected to allow for preselection.